Target center television tracking system

ABSTRACT

A target adaptive television contrast tracker for missile application wherein the adaptive feature comprises two independent linearly weighed tracking gates, one of which keeps account of positive target edge information, and the second keeps account of negative target edge information. The separation of the gates is arranged to coincide with the separation of the target edges, and the means raster position of the gates coincides with the center of the target.

Z /we TARGET CENTER TELEVISION Primary Examiner-Benjamin A. Borchelt TRACKING SYSTEM Assistant Examiner-H. A. Birmiel R V 75 Inventor: Joseph s. Brugler, Stanford, Calif. g'mfii Gwge J Rubens and [73] Assignee: The United States of America IS represented by the Secretary of the [57] ABSTRACT Navy A target adaptive television contrast tracker for mis- [22] Filed: Jan. 25, 1968 sile application wherein the adaptive feature comprises two independent linearly weighed tracking [211 App! 7o3228 gates, one of which keeps account of positive target edge information, and the second keeps account of [52] [1.8. CI. ..l78/6.8, l78/DIG. 21, 250/203 n gativ targe edge information. The separation of [51] Int. Cl. ..l-l04n 3/22 the gates is arranged to coincide with the separation of [58] Field of Search l78/6.8, 7.5 SE, th targ dg an the ans raster p n of h l78/DlG. 21; 244/317; 250/203 CT gates coincides with the center of the target.

Re'er'nces 7 UNITED STATES PATENTS 3.5l8.368 6/1970 Olson ..l78/DlG. 21

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Patented May 1, 1973 5 Sheets-Sheet 1 I N VENIY )R JOSEPH S. BRUGLE R ROY MILLER ATTORNEY.

, RIGHT AGC I Patented May 1, 1973 HORIZONTAL svuc. A

SAWTOOTH GENERATOR 8 5 Sheets-Sheet} FIG. 3.

LEFT VOLTAGE C COMPARATOR LEFT 7 noruzomu. D

IULTIVIBRATOR mam vourm: E COM PARATOR RIGHT HORIZONTAL F HULTIVIBRATOR F VIDEO ROM 6 CAMERA LEFT AGC H Tl M E VERHCAL J SYNC.

VARIABLE K WIDTH VERTICAL FIG. 4.

NULTIVIBRATOR Patented May 1, 1973 5 Sheets-Sheet 4.

uzrrl N Patented May 1, 1973 5 Sheets-Sheet 5 TARGET CENTER TELEVISION TRACKING SYSTEM BACKGROUND OF THE INVENTION This invention relates to an automatic tracking system of the type in which the positional information regarding the object to be tracked is derived by means of a television camera.

The basis of this system is a gyro-stabilized, closed circuit, television camera that furnishes the pilot an exact picture of the target and the surrounding area.

One such system is the television target tracking system disclosed in the copending patent application of Jack A. Crawford et al. Ser. No. 224,594, filed Sept. 12, 1962 and the improvements disclosed in the copending patent applications of Joseph S. Brugler et al. Ser. No. 487,635 filed Sept. 15, 1965 and Joseph S. Brugler Ser. No. 487,637 also filed Sept. 15, 1965. In the Crawford system there are two identical automatic feedback control type tracking channels for tracking in the horizontal and vertical directions, respectively, relative to a conventional television raster. The video processing section disclosed in the copending patent application of Jack A. Crawford et al. Ser. No. 224,594 electronically differentiates the video signals received from the camera and converts steps of video to short duration pulses. Full-wave rectification then creates output pulses of the same polarity, regardless of whether the change in target brightness is dark to light or light to dark. This differentiation takes place only in the direction along scanning lines. No differentiated video is obtained from the top or bottom edge of the target, only from the horizontal left and right edges. Thus, the system will center track vertically and track either the left or right edge horizontally.

SUMMARY In order to maintain a central aim point horizontally by means of contrast tracking, both target edge positions must be known to the tracker. This is accomplished by employing two detecting areas or gates, one each for the left and right horizontal edges. These gates are moved with respect to each other by the output from a separation integrator and moved to the left or right with respect to the mean raster position by the output of a second left-right integrator. Two distinct feedback loops are employed, one controlling the separation of the gates and the second controlling the position of the gates within the television raster. Updown tracking is accomplished as in the aforementioned Crawford system.

To initiate tracking, a pilot flies his plane to center the target within the cross hairs which appear on his cockpit video monitor. He then "locks-on" to the target by simultaneously freeing the tracker from its preset initial conditions, and uncaging the gyro-stabilized television camera.

The two tracking gates are initially separated slightly farther apart than the expected target width at lockon." Upon lock-on command, the television camera is uncaged and a bias signal placed upon the separation integrator causes the left gate to move to the right, and the right gate to move to the left at a constant rate. The separation of the gates is commanded to continuously change in this manner until each gate has encountered an edge having the proper contrast polarity. This is accomplished by using the difference of the left and right channel error signals as a feedback signal to stop the relative motion of the gates. When the difference of the left and right channel error signals equals the separation command bias, relative motion of the gates ceases.

Simultaneously, the sum of the left and right channel error signals is used by the left-right tracking loop to keep the target centered at the mean raster position.

The gates are then set up on the target and remain locked-on until impact. The missile flies in response to the error command signals, which are the outputs of the left-right and up-down integrators, to prevent loss of lock-on and cause impact to occur at the central aim point.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a preferred embodiment of the invention;

FIG. 1A diagrammatically illustrates the raster pattern of FIG. 1 as seen by the pilot;

FIG. 2 is a block diagram of a second embodiment of the invention;

FIG. 2A diagrammatically illustrates the raster pattern of FIG. 2 as seen by the pilot;

FIG. 3 shows waveform appearing at certain points of the horizontal tracking portion of the system of FIGS. 1 and 2;

FIG. 4 shows waveforms appearing at certain points of the vertical tracking portion of the systems of FIGS. 1 and 2;

FIG. 5 shows waveforms illustrating the operation of the horizontal discriminator-integrator-comparator servo loop; and

FIG. 6 is a block diagram of the missile and tracking system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The subject of the invention comprises the tracking unit of an automatic television tracking system which continuously aims the optical axis of a gyro-stabilized television camera at a preselected target. Referring to FIG. 6, the automatic television tracking system as illustrated includes a television camera 66 mounted for movement in a horizontal plane about a vertical axis V and for movement in a vertical plane about a horizontal axis H. Torquer motors 62 and 63 continuously operate to aim camera 66 at a preselected target 52 in response to signals 11 and 18 received from the tracking unit.

Target 52 is selected by use of monitor display 51 which is operatively connected to the tracking unit by data link 16. The tracking unit receives target brightness information from an output 15 of vidicon television camera '66. In order to synchronize the tracking unit with the deflection of the electronic beam scanning within the vidicon, horizontal deflection generator 64 and vertical deflection generator 65 emit horizontal synchronization pulses l0 and vertical synchronization pulses 19 which are fed into the tracking unit.

The operation of the two embodiments illustrated in FIG. 1 and 2 are precisely the same, however, the means for generating the cross hairs displayed to the pilot on monitor 51 is different. First, the operation of the tracker will be discussed followed by an explanation of how the various cross hairs may be generated. For simplicity, reference will be made to FIG. 1 as the cross hairs generated in accordance with that embodiment enable one to better understand the operation of the tracker.

Prior to target lock-on" the vertical servo loop and the left-right servo loop have initial conditions placed. upon them by means of switches S, and 8 These initial conditions, which are created by feedback resistors within vertical integrator 45 and left-right integrator 22, produce constant d.c. outputs V1. and V,,,, to center the tracking gates about the mean raster position.

The servo loop controlling the separation of the gates is conditioned prior to lock-on by means of switch S, and a suitable feedback resistor within separation integrator 24 to output a constant d.c. voltage This dc voltage insures that the two tracking gates will be farther apart than the expected target width at lockon! To "lock-on to a target, 52 is placed at the center of cross hairs 53 and 54 shown in FIG. 2A. This is accomplished by flying the airplane in a suitable manner. To initiate tracking, the switches S S, and S, are simultaneously thrown, closing the three servo tracking loops. Bias 14 causes left gate 60 to move to the right, and right gate 61 to move to the left until target edges of the proper light-to-dark or dark-to-light ratio are encountered. During this edge-locating" period, the gates may cross over each other.

The waveforms shown in FIG. 3, FIG. 4 and FIG. 5 give insight into the operation of the tracker. FIG. 3A shows the waveform representative of the output of the horizontal sync 64. The horizontal sync pulses, which occur at times t, and t, trigger a sawtooth generator 20 whose output waveform is shown in FIG. 3B. Left gate 60 which exists from time 1, until I, is initiated by the output spike of left voltage comparator 23 as shown in FIG. 3C. As shown in FIG. 3B, the left voltage comparator will generate a voltage spike whenever the output of sawtooth generator 20 and the dc. voltage equal to the difference between the outputs of the left-right integrator V, and the separation integrator V. coincide in time. Similarly, right voltage comparator 25 will generate a voltage spike whenever the output of sawtooth generator 20 and the dc voltage equal to the sum of V, and V, coincide in time.

The output of left voltage comparator 23 triggers left horizontal monostable multivibrator 28 producing an output during time interval t, to t, and the output of right voltage comparator 25 produces a similar reaction from right horizontal monostable multivibrator 29 which results in an output during time interval 1, to l The outputs from left horizontal multivibrator 28 and right horizontal multivibrator 29 open left window" gate 38 and right "window" gate 39, respectively.

Video from camera 66 changes in output as the electron beam of the television camera sweeps across the raster. Thus, when atarget is perceived by the camera, the output is either increased or decreased according to the target contrast ratio. FIG. 3G shows an increase in output, however, it should be noted that a decrease in output may also be perceived depending on whether the target-to-background contrast ratio is greater or less than zero. Assuming that the tracker is in operation, the time at which the camera first detects the target will occur somewhere between time I, and I which is during the time interval when left horizontal multivibrator 28 is in operation and left window" gate 38 is open. As the beam further sweeps across the vidicon horizontally, the right edge of the target will leave the field of view at time 1-,, which during tracking occurs somewhere between time t, and l and is during the period of operation of right horizontal monostable multivibrator 29 which opens right window" gate 39. The raw video from camera 15 enters differentiator 34 to emerge as a positive voltage spike at time 1;, and a negative voltage spike at time I, if the target is of such brightness that the video output increases as shown in FIG. 3G. The output of differentiator 35 will be a negative voltage spike at time 1;, and a positive voltage spike at time t, if the target produces a video output 15 which is the inverse of that shown in FIG. 3G. The polarity of the pulses shown in FIG. 3G depends upon whether the target is lighter or darker than its environment as previously explained.

If one assumes a video output 15 as shown in FIG. 3G, the output from positive half wave rectifier 36 will pass through left "window gate 38 and the output of negative half wave rectifier 37 will pass through right window gate 39. If the video output from camera 15 is the inverse from that shown in FIG. 3G, the output of positive half wave rectifier 36 will pass through right window" gate 39 and the output of negative half wave rectifier 37 will pass through left window gate 38. This has no effect whatsoever on the operation of the tracker.

The output from positive half-wave rectifier 36 next passes through left window gate 38 to left video amplifier 40 and left automatic gain control 42 which produces a spike of uniform height no matter what magnitude of change in target brightness value is transmitted from camera 66. Similarly, the output from negative half-wave rectifier 37 passes through right window gate 39 to right video amplifier 41 and right automatic gain control 43. The output of left automatic gain control 42, which occurs at time 1 and right automatic gain control 43, which occurs at time t, are the amplified and clipped outputs of positive and negative half wave rectifiers 36 and 37 as shown in FIGS. 3H and 31, respectively. The output pulses of left automatic gain control 42 and right automatic gain control 43 are in turn monitored by left horizontal discriminator 30 and right horizontal discriminator 31.

A full explanation of the working of a discriminator of the type shown in this application may be found in the copending patent application of Joseph S. Brugler et al. Ser. No. 487,635 filed Sept. 15, 1965.

Attention is now directed to FIG. 5A. The outputs of left horizontal discriminator 30 and right horizontal discriminator 31 are represented by e, and e, respectively. The time t, occurs shortly after tracking is initiated. At time t,, the target is centered between the two tracking gates and each edge of the target is centered within each of the two gates, thus the magnitude of the output 2, is equal to the magnitude of the output 2,. For convenience, this initial output is represented by a pulse having a magnitude 2X. The outputs of the left horizontal discriminator e, and right horizontal discriminator e are added and subtracted; the sum then being integrated by left-right integrator 22 and the difference being integrated by separation integrator 24. The outputs of left-right integrator 22, V, and the output of separation integrator 24, V, thus are essentially d.c. voltages which change level in response to the command received from the left and right horizontal discriminators.

Because the left-right tracking loop and the separation tracking loop are independent, shifts to the left or right of the mean raster position and separation of the gates caused by the target blooming as the missile approaches it can be independently accounted for. A pulse e, occurs once each frame as the scanning lines cross through gate 60 and a pulse e, occurs once each frame as the scanning lines cross through gate 61. The pulses shown in FIG. 5A at the various times t,, t and t, are representative of the series of pulses which are generated by the left and right horizontal discriminators for various target positions within gates 60 and 61.

The pulse occurring at t, is representative of a time at which the target is precisely centered within each tracking gate. 1f the left edge of target 52 were to move toward the left of tracking gate 60 the magnitude of the output of the left discriminator e, would increase and if the left edge of the target 52 were to move toward the right edge of the gate 60 the magnitude of e, would decrease. Similarly, if the right edge of target 52 were to move toward the left within right gate 61 the value of e, would decrease and if the right edge were to move toward the right of gate 61 the value of e, would increase. At time t, the target 52 has moved to the left with respect to the mean raster position and the relative size of the target has remained the same, therefore the separation between gates 60 and 61 remains the same. The relative leftward shift of the target causes the value of e, to increase and the value of e, to decrease. Referring to Table 5B, the relative change in position with respect to mean raster position AV, equals 2 and the relative change in separation AV, remains zero. The values of Table 513 may be calculated for time I, as follows:

At time t, the target has remained in the same position with respect to the mean of the television raster, how ever, the target has bloomed" or increased in relative size to provide an error signal to cause gates 60 and 61 to move farther apart. At this time the value of I, has increased as the left edge of the target has moved to the left within gate 60 and the value of I, has increased to 3X as the right edge of the target has moved toward the right side of gate 61. Taking the sum and difference of e; and e,; the left-right position error after integration, V,, remains the same and the separation error after integration, V,, increases by two units. At time 1,. target 52 has shifted relatively with respect to the mean raster position and the target increased in relative size. This change in position is reflected in an increase in e, to

value 3X and a return to value 2X in e, Taking the sum and difference of e, and e, and integrating the result yields a change in V, of plus one l indicating a leftward shift with respect to mean raster position and a change in separation V, of plus one l) causing the gates to move farther apart. As the television camera continuously sweeps the electron beam over the vidicon face, the tracker continuously corrects by generating various levels of e, and e which in turn generate changing left-right position errors V, and separation errors V,. Position error V, is fed to horizontal torquer motor 62 by data link 11 to provide a servo follow-up to the cross hair tracking and maintain aim of the camera at its target. The cross hairs track at a relatively high rate, while the camera follows-up slowly, much the same way as a human eyeball sights an object, and the head then turns toward the direction at which the eye is looking.

Attention is now directed to FIG. 5C. Left voltage comparator 23 and right voltage comparator 25 will trigger and produce an output pulse whenever the sum and difference of V, and V, coincide in time with the output of sawtooth generator 20. The difference of V, and V, is used to trigger left voltage comparator 23 at time I, and the sum of V, and V, is used to trigger right voltage comparator 25 at time t,. Thus, the difference of V, and V, controls the position of the left "window" gate while the sum of V, and V, controls the position of the right window gate. If the value of V, is held constant, an increase in the value of V,, which corresponds to a relative leftward movement of the target within the gate, moves the gate to the right to counteract the leftward movement. This can be seen as an increase in the level of V, which will cause the intersection in time of the sum and difference to occur later, thus causing both the left and right gates to appear farther to the right with respect to the mean raster position. If V, is held constant, then an increase in V, will move the gates farther apart and a decrease will move the gates closer together. Just prior to lock-on there is a constant output from the separation integrator, V,, separating gates 60 and 61 slightly farther apart than expected target width at lock-on."

Referring to F [0. 2A, the pilot has displayed on his monitor a set of cross hairs for use in aligning the missile on the target. The plane is flown so the cross hairs intersect the center of the target as closely as possible. In this embodiment, cross hair 54, which is generated by horizontal cross hair voltage comparator 21, remains at the center of the two tracking gates 60 and 61 which are shown during tracking in FIG. 1A. When the pilot initiates tracking, gates 60 and 61 which are separated move toward each other in response to the command from bias 14. As the left and right discriminators begin to output signals the value of the difference between e, and e, which is subtracted from the command bias 14 quickly overcomes the small bias 14 to initiate operation of separation integrator 24. A full discussion of the generation of the cross hairs 53, 54, 60, 61 and 59 follows the discussion of the up-down tracking loop.

Attention is directed to FIG. 4 as an aid in understanding the operation of the up-down tracking loop. Television camera 66 generates a vertical sync pulse 19 which pulse occurs once each raster and is shown in FIG. 4.]. The vertical sync pulse shown in FIG. 4.] is used to trigger vertical variable width monostable multivibrator 46 the output of which is shown in FIG. 4K. The spike pulse shown in FIG. 4K at time t is created by vertical variable width multivibrator 46 which consists of a monostable multivibrator having a width in time from i until I coupled with a negative half wave rectifier to produce only the output spike shown in FIG. 4K. This output spike from vertical variable width monostable multivibrator 46 triggers vertical monostable multivibrator 49, which is shown in FIG 4L. Vertical multivibrator 49 is on during the time interval t to t,,, which time interval corresponds to the vertical opening of left window" gate 38 and right window" gate 39. The time interval r to 1,: also corresponds to the interval during which upper vertical discriminator 50A and lower vertical discriminator 50B operate. The processed video which emanates from left automatic gain control 42 and right automatic gain control 43 is summed by summer 44 and the output is directed onto the upper and lower vertical discriminators. The upper vertical discriminator outputs a positive series of pulses e which are similar to the error signal e, outputted from left horizontal discriminator 30. Similarly, lower vertical discriminator 50B emits a negative series of pulses e which are similar to the pulses emitted by right horizontal discriminator 31. If the upper edge of the target is displaced higher or lower, that is early or late with respect to the vertical target gate, the value e will either be increased or decreased proportionate to the amount of displacement. In the same manner, the lower vertical discriminator generates an error signal e which signal has a magnitude proportional to the position of the lower target edge with respect to the gate. The magnitude of e, and e,, are summed and then integrated to produce a vertical signal V,,. Vertical error signal V repositions the cross hairs on target and commands the vertical torquers and servos via data link 18 to fly the missile in accordance with the tracking error.

The generation of cross hairs 59, 60, and 61 may be explained with reference to FIG. I. The output of left horizontal multivibrator 28, as shown in FIG. 3D, occurs from time 1, until time and the output of right horizontal multivibrator 29 as shown in FIG. 3F occurs from time I, until time t,,. The time intervals t, and t and t, to t, correspond to the respective times during which left window" gate 38 and right window" gate 39 are open. Thus, the leading and trailing edges of the pulses as shown in FIGS. 3D and 3F correspond to the open" and close" of each gate 60 and 61 shown in FIG. IA. A left cross hair differentiator 57 differentiates the output of left horizontal multivibrator 28, thus a voltage spike is inserted into the video monitor picture IA by mixer 35 at time intervals I, and I thereby forming cross hairs 60 on video display 51. Similarly, the right cross hair differentiator 56 differentiates the output of the right horizontal multivibrator 29 thereby creating voltage spikes at times I and t, which generate cross hairs 61 on television monitor display 51 when passed through mixer 35. Vertical cross hairs 59 are generated by vertical cross hair differentiator 58, the output of which also passes through mixer 35 and then through the pilot's display 51 via data link 16. Mixer 35 additionally provides the means for placing the video output from camera 15 upon the pilot's display 51 through data link 16.

Turning now to FIG. 2, the generation of cross hairs 53 and 54 may be explained. FIG. 3D illustrates the output of left horizontal multivibrator 28 which occurs at time I5 corresponding to the intersection in time of the outputs of sawtooth generator 20 and left-right integrator 22 (V,). Horizontal cross hair voltage comparator 21 outputs a voltage spike at time I, which time corresponds to the mean position of the cross hairs on television screen 51. This spike, when mixed with video signal 15 in mixer 35, produces the cross hair 54 shown on the pilot's display 51. Cross hair 53 is generated as follows: The output of the vertical variable width multivibrator 46, as shown in FIG. 4K, occurs at time t Vertical cross hair monostable multivibrator 47 has a time constant equal to one half the constant of vertical monostable multivibrator 49. The output of vertical cross hair multivibrator 47 is differentiated by vertical cross hair differentiator 48 which creates a positive voltage spike at time t and a negative voltage spike at time 1,, plus one half the time difference between 1, and t The output of vertical cross hair differentiator 48 is passed through negative half-wave rectifier 67 to filter out the positive pulse which occurs at time I leaving the negative pulse to generate cross hair 54. The output of negative half-wave rectifier 67 is combined with the video from camera 15 and the output of the horizontal cross hair voltage comparator 21 by mixer 35 to produce pilot's display 51 as shown in FIG. 2A.

Various other means of cross hair generation ma be accomplished .-without departing from the spirit and scope of the invention.

Many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

l. A television contrast system for tracking targets within the field of view of a television camera for hom ing missile application comprising:

television camera means for producing a visual display ofa field of view about its optical axis;

means for generating and inserting a first tracking area or gate into said visual display;

means for generating and inserting a second tracking area or gate into said visual display;

means responsive to said visual display for positioning the first tracking area or gate about a first horizontal edge of a target within the field of view of said television camera; means responsive to said visual display for positioning the second tracking area or gate about a second horizontal edge of a target within the field of view ofsaid television camera; and

means responsive to movement of said target within each of the tracking areas or gates for maintaining said first and second horizontal edges of the target within said first and second tracking areas or gates so that the optical axis of said television camera is always aligned with the centroid of said target.

2. The system of claim 1 wherein the means for generating, inserting and positioning said first tracking area or gate about a first horizontal edge of said target comprises:

a first horizontal monostable multivibrator having triggering means responsive to a first voltage spike; and

a first voltage comparator to output a first voltage spike to trigger said first monostable multivibrator in response to the coincidence in time of the waveforms resulting from:

a sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and

a first horizontal position signal generated by said means for aligning the optical axis of said television camera with the centroid of the target.

3. The system of claim 2 wherein the means for generating, inserting an positioning said second tracking area or gate about a second horizontal edge of said target comprises:

V a second horizontal monostable multivibrator having triggering means responsive to a second voltage spike; and

a second voltage comparator to output a second voltage spike to trigger said second monostable multivibrator in response to the coincidence in time of the waveforms resulting from:

said sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and

a second horizontal position signal generated by said means for aligning the optical axis of said television camera with the centroid of the target.

4. The system of claim 3 wherein the means for generating said first and second error signals for aligning the optical axis of said television camera with the centroid of the target comprises:

a first horizontal discriminator which produces a first horizontal error signal proportional to the position of said first edge of the target within said first tracking area or gate;

a second horizontal discriminator which produces a second horizontal error signal proportional to the position of said second edge of the target within said second tracking area or gate;

means for summing said first and second horizontal error signals and integrating the sum to produce a left-right error signal;

means for subtracting said second horizontal error signal from said first horizontal error signal and integrating the difference to produce a separation error signal;

means for subtracting the separation error signal from the left-right error signal to produce a first horizontal position error signal to trigger said first voltage comparator when said first horizontal position error signal coincides in time with the output of the sawtooth generator; and

means for summing the left-right error signal and the separation error signal to produce a second horizontal position error signal to trigger said second voltage comparator when said second horizontal position error signal coincides lll time with the output of the sawtooth generator.

5. The system of claim 4 and further including means for generating cross hairs which define the tracking area or gate.

6. The system of claim 5 wherein the means for generating said cross hairs comprises:

a first differentiator which operates on the signal produced by the first horizontal monostable multivibrator;

means for combining the output of said first differentiator with the output of said television camera;

a second differentiator which operates on the signal produced by the second horizontal monostable multivibrator; and

means for combining the output of said second differentiator with the output of said television camera.

7. The system of claim 5 wherein the means for generating said cross hairs comprises:

a third horizontal voltage comparator to output a third voltage spike in response to the coincidence in time of the waveforms resulting from: the sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and the left-right error signal; and

means for combining the output of said third horizontal voltage comparator with the output of said television camera. 

1. A television contrast system for tracking targets within the field of view of a television camera for homing missile application comprising: television camera means for producing a visual display of a field of view about its optical axis; means for generating and inserting a first tracking area or gate into said visual display; means for generating and inserting a second tracking area or gate into said visual display; means responsive to said visual display for positioning the first tracking area or gate about a first horizontal edge of a target within the field of view of said television camera; means responsive to said visual display for positioning the second tracking area or gate about a second horizontal edge of a target within the field of view of said television camera; and means responsive to movement of said target within each of the tracking areas or gates for maintaining said first and second horizontal edges of the target within said first and second tracking areas or gates so that the optical axis of said television camera is always aligned with the centroid of said target.
 2. The system of claim 1 wherein the means for generating, inserting and positioning said first tracking area or gate about a first horizontal edge of said target comprises: a first horizontal monostable multivibrator having triggering means responsive to a first voltage spike; and a first voltage comparator to output a first voltage spike to trigger said first monostable multivibrator in response to the coincidence in time of the waveforms resulting from: a sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and a first horizontal position signal generated by said means for aligning the optical axis of said television camera with the centroid of the target.
 3. The system of claim 2 wherein the means for generating, inserting an positioning said second tracking area or gate about a second horizontal edge of said target comprises: a second horizontal monostable multivibrator having triggering means responsive to a second voltage spike; and a second voltage comparator to output a second voltage spike to trigger said second monostable multivibrator in response to the coincidence in time of the waveforms resulting from: said sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and a second horizontal position signal generated by said means for aligning the optical axis of said television camera with the centroid of the target.
 4. The system of claim 3 wherein the means for generating said first and second error signals for aligning the optical axis of said television camera with the centroid of the target comprises: a first horizontal discriminator which produces a first horizontal error signal proportional to the position of said first edge of the target within said first tracking area or gate; a second horizontal discriminator which produces a second horizontal error signal proportional to the position of said second edge of the target withIn said second tracking area or gate; means for summing said first and second horizontal error signals and integrating the sum to produce a left-right error signal; means for subtracting said second horizontal error signal from said first horizontal error signal and integrating the difference to produce a separation error signal; means for subtracting the separation error signal from the left-right error signal to produce a first horizontal position error signal to trigger said first voltage comparator when said first horizontal position error signal coincides in time with the output of the sawtooth generator; and means for summing the left-right error signal and the separation error signal to produce a second horizontal position error signal to trigger said second voltage comparator when said second horizontal position error signal coincides in time with the output of the sawtooth generator.
 5. The system of claim 4 and further including means for generating cross hairs which define the tracking area or gate.
 6. The system of claim 5 wherein the means for generating said cross hairs comprises: a first differentiator which operates on the signal produced by the first horizontal monostable multivibrator; means for combining the output of said first differentiator with the output of said television camera; a second differentiator which operates on the signal produced by the second horizontal monostable multivibrator; and means for combining the output of said second differentiator with the output of said television camera.
 7. The system of claim 5 wherein the means for generating said cross hairs comprises: a third horizontal voltage comparator to output a third voltage spike in response to the coincidence in time of the waveforms resulting from: the sawtooth generator responsive to a horizontal synchronization pulse generated by said television camera; and the left-right error signal; and means for combining the output of said third horizontal voltage comparator with the output of said television camera. 